The exhaust gas of a diesel engine, gasoline engine, or other internal combustion engine, for example, contains carbon monoxide (CO), unburned fuel (HC), nitrogen oxides (NOx), particulate matter (PM), or other constituents. The internal combustion engine has an exhaust purification system attached to it to remove these constituents.
As one method for removing nitrogen oxides, it has been proposed that an NOx storage reduction catalyst be arranged in an engine exhaust passage. An NOx storage reduction catalyst stores NOx when an air-fuel ratio of exhaust gas is lean. When a stored amount of NOx reaches an allowable amount, the air-fuel ratio of the exhaust gas is made rich or a stoichiometric air-fuel ratio so that the stored NOx is released. The released NOx is reduced by a reducing agent, such as the carbon monoxide, etc., which is contained in the exhaust gas.
The exhaust gas of an internal combustion engine sometimes contains sulfur oxides (SOx). An NOx storage reduction catalyst stores SOx simultaneously with storing NOx. If SOx is stored, the storable amount of NOx falls. In this case, in the NOx storage reduction catalyst, so-called “sulfur poisoning” occurs.
To keep an NOx storage reduction catalyst from storing SOx, there is known an exhaust purification system which arranges at an upstream side of the NOx storage reduction catalyst an SOx trap catalyst which has a function of removing the SOx.
Japanese Patent Publication (A) No. 2000-314309 discloses an exhaust purification system of an internal combustion engine which is provided with a storage reduction type NOx catalyst provided in an exhaust passage of a lean combustion type internal combustion engine, an SOx trap material which is provided at an upstream side of the storage reduction type NOx catalyst and in the vicinity of the internal combustion engine, a bypass passage which guides exhaust gas at an upstream side of the storage reduction type NOx catalyst to a downstream side of the storage reduction type NOx catalyst, and means for switching between the bypass passage and an exhaust pipeline to the storage reduction type NOx catalyst.